The present invention relates to a method for manufacturing high-purity 2,2'-di(4-hydroxyphenyl)-propane, also called bisphenol A. The bisphenol A is used as a feedstock for producing resins, principally epoxy and polycarbonate resins, including the optical polycarbonate grade where especially high purity and good colouration are required.
Bisphenol A is obtained from a reaction of phenol and acetone in the presence of a strong-acid cation exchanger as catalyst. The post-reaction mixture contains, besides the bisphenol A, unreacted phenol, acetone and water, or catalyst and water, and by-products, mainly 2-(4-hydroxy phenol)-2'-(2-hydroxyphenyl)-propane, i.e., ortho-para isomer of bisphenol A, the so-called o,p-isomer, 2-2-4-trimethyl-4-(4-hydroxyphenyl)-chromane, i.e. the so-called Dianin compound, trisphenols, polyphenols and coloured compounds. Depending on the methods selected for further processing the post-reaction mixture, recovery and purification of bisphenol A, the final product may contain some amount of by-products which deteriorate its purity and colouration and unfavourably affect further processing.
A number of methods for removing the impurities are known in the art. The most commonly used method is to remove acetone and water, or catalyst, acetone, water and part of phenol from the post-reaction mixture followed by recovery, by crystallization, of bisphenol-A/phenol adduct which is then separated by distillation into raw bisphenol A and phenol. The mother liquor remaining after adduct recovery, which contains primarily phenol and some by-products and dissolved bisphenol A, can be recycled to the reaction system. In a multistage process, the bisphenol A is recovered by thermal catalytic decomposition of the by-products of the principal process technology.
The Polish Patent No. P-268 149 describes a process to manufacture bisphenol A where the feed is obtained by mixing the recycled phenolic liquor with part of the post-reaction mixture collected in the reaction system. The initial concentration of bisphenol A in this feed is 12-20% wt of bisphenol A, the content of bisphenol isomers in the total amount of by-products is less than 25% of the by-products and the phenol-to-acetone mole ratio is (5-30):1. The reaction is conducted at 60.degree.-95.degree. C. in the presence of a catalyst such as a mixture of macro- and microporous cation exchanger resin in the weight ratio of (0.05-0.5):1, respectively. This reaction results in a mixture containing bisphenol A at a high concentration, i.e. 21-35% wt, and 12-24% wt of by-products. Owing to the phenol liquor being recycled to the reaction system, the quantity of the impurities is increased. In order to prevent cumulation of undesirable substances in the process, the impurities are removed by withdrawal of part of the liquor stream from the reaction system and processing it as required, to recover a suitable amount of bisphenol A.
The European Patent No 3328870 describes a method to remove impurities from bisphenol A consisting of the addition of water to the post-reaction mixture following removal of the catalyst therefrom, and vacuum evaporation of the water and part of the phenol from the mixture, resulting in bisphenol-A/phenol adduct crystallization. The separated water is then contacted, as a mixture with phenol and the impurities, with a weak-basic ion-exchange resin prior to being recycled to the crystallized adduct and then on with the crystallized adduct, for further treatment. The substantial disadvantage of the method is that only a small percentage of the impurities are removed from the post-reaction mixture.
The UK Patent No 1565667 describes a method to recover bisphenol A and remove coloured impurities from all or part of the recycled mother liquor stream by contacting it with an adsorber bed in the form of a cation-exchange resin. This permits some impurities present in the mother liquor to be removed before it is recycled to the reaction system. Obviously, the volume of mother liquor which can be purified using this method is limited; furthermore, the adsorber does not adsorb all the impurities to be removed and additional regeneration is required, i.e. washing, drying and removal of impurities from the washing liquid. Thus, part of the impurities are transferred to the reaction system with the liquor being recycled, the result of which is recycled bisphenol A lower in quality than that produced in the reaction of phenol with acetone.
Some other methods for removing impurities and recovering bisphenol A from all or part of the post-crystallization liquor arising from the step of bisphenol-A/phenol adduct separation or from process by-products produced in the process for obtaining bisphenol A from their catalytic decomposition while distilling off the product being formed and recycling the waste products to the reaction are also known to those skilled in the art.
According to Polish Patents No 113641 and 103054, impurities are removed and bisphenol A is obtained with high efficiency by a method where phenol, in the presence of cation-exchanger resin catalyst, is reacted with intermediates, such as p-isopropenylphenol, o-isopropenylphenol and their dimers, obtained by thermal catalytic decomposition of bisphenol A process by-products and introduced to the reaction in phenolic solution, the said phenolic solution of synthesis by-products being dewatered post-crystallization liquor or fractions formed during distillation of the dewatered post-crystallization liquor. The reaction of phenol with the products of thermal catalytic decomposition of the bisphenol A process by-products is, according to the cited Patents, conducted in a separate reactor, under different conditions than those accompanying the phenol-acetone reaction, preferably in the presence of 5-20% by weight of water.
The methods, as described above, do not show good efficiency; part of the undesirable impurities are transferred, with the adduct, into subsequent steps of bisphenol A process.
The European Patent No 0332203 describes a method to obtain high-purity bisphenol A, comprising a principal process and a so-called sub-process. In the principal process, phenol is made to react with acetone and the post-reaction mixture is treated to obtain a solution with a desirable concentration of bisphenol A: this is the so-called step I of concentration control and is followed by step I of bisphenol A/phenol adduct crystallization, step I of adduct separation from the mother liquor and step I of phenol removal from the adduct to obtain a high-purity bisphenol A. The sub-process consists of the following: step II of phenol reaction with p-isopropenylphenol, step II of bisphenol A concentration control, step II of bisphenol-A/phenol adduct crystallization, step II of adduct crystalline fraction II separation from mother liquor II, and step II of mother liquor II processing to obtain p-isopropenylphenol and phenol, the mother liquor I from the principal process being fed to the sub-process. The adduct crystalline fraction from the sub-process is fed to the principal process.
The European Patent No EP 0332203 also claims another variant of the process to obtain a high-purity bisphenol A. The principal process comprises the following: step I of reaction where phenol is made to react with acetone and catalyst is removed, step of crystallization where bisphenol-A/phenol adduct is recovered, step of crystalline adduct separation from mother liquor, and step of phenol removal from crystalline adduct.
The sub-process consists of the following: phenol reaction with p-isopropenylphenol in the presence of an acid catalyst and catalyst removal to obtain phenolic solution II, removal of phenol from phenolic solution II to obtain crude bisphenol A, separation of low- and high-boiling substances from raw bisphenol A by distillation to obtain distilled bisphenol A, and processing of the separated low- and high-boiling substances to obtain p-isopropenylphenol and phenol. The mother liquor from the principal process is fed to the sub-process and the distilled bisphenol A from the sub-process is fed to the principal process.
The resulting bisphenol A shows high purity due to "dilution" of impurities by introduction of adduct or bisphenol A, recovered in the sub-process, to the principal process as a starting material. The main disadvantage of the process to obtain high-purity bisphenol A according to Patent No 0332203 is the complexity created by the introduction of the sub-process comprising a number of steps of a complexity comparable to that of the principal process. This results in a considerable increase in the investment and operating costs of the bisphenol A plant where the process is used.